Conductive polymers have long been known in the art, including polyacetylene, polypyrrole, poly(para-phenylene), and derivatives thereof. While in some cases exhibiting metallic-like conductivity, highly conductive polymers have been limited in their practical applications because they are typically chemically unstable in use, and virtually intractable, being unsuited for either solution or melt processing. All conductive polymers require acid or oxide functionality, usually referred to as doping, to achieve their high conductivities.
Polyaniline (PANI) stands out among conductive polymers in that it is known in the art to be chemically stable and readily soluble in conventional, environmentally friendly solvents, and thus offers the possibility for employing ordinary means known in the art forming coatings, films and sheets, fibers, printed patterns, and so forth.
Conductive PANI is described in great detail in Chiang et al, Synthetic Metals, 13 (1986), pp. 193-205. Chiang et al disclose numerous PANI compositions, identifying the protonic acid doped emeraldine nitrogen base salt, as the most highly conductive form, with a conductivity of 5 S/cm. This conductivity remains well below the 102 S/cm range characteristic of certain other conductive polymers, and which represents practical threshold conductivity for widespread utility in electronics.
Levon et al, Polymer 36, pp 2733ff (1995) and Ahiskog et al, Synthetic Metals, 69, pp 135ff (1995) disclose formation of the PANI nitrogen base salt at elevated temperature by combining with liquid organic acids such as dodecylbenzenesulfonic acid (DBSA).
There is considerable incentive to find a way to enhance the conductivity of PANI while preserving the desirable chemical stability and processibility thereof. Specifically, a PANI composition exhibiting a conductivity of ca. 102 S/cm may be a highly preferred material for important applications in electronics.
It is known in the art to combine PANI with inorganic fillers, including conductive fillers such as graphite, metal fibers, and superconducting ceramics, see for example Jen et al, U.S. Pat. No. 5,069,820.
Carbon nanotubes are a relatively new form of matter related to C60 the spherical material known popularly as “Buckminster Fullerene” While new, carbon nanotubes have elicited much interest because of their unusual structure and are available commercially. They are described in considerable detail in Carbon Nanotubes and Related Structures, by Peter J. F. Harris, Cambridge University Press, Cambridge, UK (1999).
Composites of conductive polymers and carbon nanotubes in the form of films are disclosed in Coleman et al, Phys. Rev. B 58 (12) R7492ff (1998), Chen et al, Advanced Materials 12 (7) 522 ff (2000), and Yoshino et al, Fullerene Sci. Tech. 7 (4) 695ff (1999).
Coleman et al, op.cit. discloses composites of poly(p-phenylenevinylene-co-2,5-dioctoxy-m-phenylenevinylene) (PMPV) with carbon nanotubes produced by an electric arc procedure. Mass fractions of nanotubes plus residual soot ranged from ca. 0.5-35%. Films were spin-coated onto a platinum surface from a toluene solution. Conductivity is shown to exhibit a six order of magnitude increase between ca. 4% and ca 9% nanotubes.
Also disclosed in Coleman et al, op.cit., is a failed attempt to make a similar composite with PMMA. The failure is said to result from molecular conformational causes.
Chen et al, op.cit., disclose composite films of nanotubes and polypyrrole. Both films and coated nanotubes are disclosed. The nanotubes are shown to enhance the conductivity of the polypyrrole. The films are deposited by exposing various substrates to a solution of pyrrole and nanotubes followed by electropolymerization of the pyrrole in situ on the substrate, thus entrapping the nanotubes within the polymer matrix. Chen also employs arc-grown nanotubes.
Yoshino et al, op.cit., disclose composites of poly(3-hexylthiophene) (PAT6) and nanotubes produced by chemical vapor deposition and purified. The nanotubes were dispersed in hexene and mixed with the chloroform solution of the polymer. Films were formed by casting on a quartz plate. A ca. 4 order of magnitude change in conductivity was observed between a volume fraction of ca. 1% to ca. 10%, with the percolation threshold estimated to be at ca. 5.9%.
Laser thermal ablation image transfer technology for color proofing and printing is described in Ellis et al, U.S. Pat. No. 5,171,650 and elsewhere. Similar methods are in current commercial use in the printing and publishing businesses.